Electrochemical and Structural Investigation of Calcium Substituted Monoclinic Li3V2(PO4)3 Anode Materials for Li-Ion Batteries

In this work, the effect of Li+ substitution in Li3V2(PO4)(3) with a large divalent ion (Ca2+) toward lithium insertion is studied. A series of materials, with formula Li3-2xCaxV2(PO4)(3)/C (x = 0, 0.5, 1, and 1.5) is synthesized and studied in the potential region 3-0.01 V versus Li+/Li. Synchrotron diffraction demonstrates that Li3V2(PO4)(3)/C has a monoclinic structure (space group P2(1)/n), while Ca1.5V2(PO4)(3)/C possesses a rhombohedral structure (space group R-3c). The intermediate compounds, Li2Ca0.5V2(PO4)(3)/C and LiCaV2(PO4)(3)/C, are composed of two main phases, including monoclinic Li3V2(PO4)(3)/C and rhombohedral Ca1.5V2(PO4)(3)/C. Cyclic voltammetry reveals five reduction and oxidation peaks on Li3V2(PO4)(3)/C and Li2Ca0.5V2(PO4)(3)/C electrodes. In contrast, LiCaV2(PO4)(3)/C and Ca1.5V2(PO4)(3)/C have no obvious oxidation and reduction peaks but a box-type voltammogram. This feature is the signature for capacitive-like mechanism, which involves fast electron transfer on the surface of the electrode. Li3V2(PO4)(3)/C undergoes two solid-solution and a short two-phase reaction during lithiation and delithiation processes, whereas Ca1.5V2(PO4)(3)/C only goes through capacitive-like mechanism. In operando X-ray absorption spectroscopy confirms that, in both Li3V2(PO4)(3)/C and Ca1.5V2(PO4)(3)/C, V ions are reduced during the insertion of the first three Li ions. This study demonstrates that the electrochemical characteristic of polyanionic phosphates can be easily tuned by replacing Li+ with larger divalent cations.